<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <title>For循环优化</title>
</head>

<body>
  <p class="btn">add</p>
  <p class="btn">add</p>
  <p class="btn">add</p>
  <p class="btn">add</p>
  <p class="btn">add</p>
  <p class="btn">add</p>
  <p class="btn">add</p>
  <p class="btn">add</p>
  <p class="btn">add</p>
  <script>
    var aBtns = document.getElementsByClassName('btn');

    for (var i = 0; i < aBtns.length; i++) {
      console.log(i);
    }

    //值固定并数值较大(优)
    for (var i = 0, len = aBtns.length; i < len; i++) {
      console.log(i);
    }
  </script>
</body>

</html>